tsql如何获取具有第一个和结束日期时间的唯一行

当我单独运行子查询时，我会为每一行获取唯一的日期，但同时会得到重复的行。我该如何更正

SELECT a.*
    ,b.*
FROM (
    SELECT DISTINCT serial_number_id
        ,s_start_dttm
        ,x_dept_id AS dept_a
        ,x_dept_name AS dept_name_a
        ,event_type_c AS event_type_a
        ,evnt.NAME AS event_name_a
    FROM TABLE x
    WHERE event_type_c = 3
    ) a
    ,(
        SELECT DISTINCT serial_number_id
            ,s_end_dttm
            ,x_dept_id AS E_x_DEPT_ID
            ,x_DEPT_NAME AS E_x_DEPT_NAME
            ,event_type_c AS event_type_b
            ,event_name AS event_type_b
        WHERE event_type_c = 4
        ) b
WHERE a.serial_number_id = b.serial_number_id

---

请添加一些测试数据和预期输出。请尝试添加和a.dept\u a=b.E\u x\u dept\u ID